Copolymers of tetrahydrofuran (THF) and at least one other cyclic ether, also known as copolyether glycols, are known for use as soft segments in polyurethanes and other elastomers, particularly where reduced crystallinity imparted by the cyclic ether may improve certain dynamic properties of polyurethane and other elastomers which contain such a copolymer as a soft segment. Examples of cyclic ethers used for this purpose are ethylene oxide and propylene oxide.
The copolymers of THF and at least one other cyclic ether are well known in the art. Their preparation is disclosed, for example, by Heinsohn et. al. in U.S. Pat. No. 4,163,115, by Pruckmayr in U.S. Pat. Nos. 4,120,903 and 4,139,567, and U.S. Pat. No. 4,153,786. Such copolymers can be prepared by any of the known methods of cyclic ether polymerization, described for instance in “Polytetrahydrofuran” by P. Dreyfuss (Gordon & Breach, N.Y. 1982). Such polymerization methods include catalysis by strong proton or Lewis acids, by heteropoly acids, as well as by perfluorosulfonic acids or acid resins. In some instances it may be advantageous to use a polymerization promoter, such as a carboxylic acid anhydride, as disclosed in U.S. Pat. No. 4,163,115. In these cases the primary polymer products are diesters, which need to be hydrolyzed in a subsequent step to obtain polyether glycols.
In these processes, crude product mixtures will contain byproduct oligomeric cyclic ethers of ethylene oxide and tetramethylene oxide units with molecular weight from about 188 to 500 (OCE), which are relatively stable compounds. When feedstock to such processes comprise THF and ethylene oxide, yield of OCE ranges, for example, from about 5 to about 25 wt % depending on the ethylene oxide to THF feedstock ratio. If the conditions are right in such processes conducted in the presence of acid catalysts, some depolymerization occurs, but at very slow rates unless appropriate catalyst and sufficient heat is applied. In any event, when such depolymerization progresses, higher molecular weight copolyether forms with higher viscosity leading to the formation of tars. For example, U.S. Pat. No. 4,202,964 shows that the OCE content of a copolymer product can be reduced by contacting the product with cationic exchange resin at certain conditions. A serious disadvantage to such a method is that open glycol chains in the copolymer product are deoligomerized with significant impact on product molecular weight distribution.
A depolymerization mechanism is shown in, for example, U.S. Pat. No. 6,429,321 where a mixture containing polytetrahydrofuran derivative is depolymerized at increased temperature over catalyst comprising zeolite Beta. Another such process mechanism is shown in U.S. Pat. No. 3,925,484 where polytetramethylene ether glycol (PTMEG) is said to be depolymerized at elevated temperature over cross-linked acid form ion exchange resin. U.S. Pat. No. 4,115,408 shows a process for depolymerizing PTMEG in which effluent containing same is heated with sulfuric acid at high temperature.
U.S. Pat. No. 4,363,924 shows use of a bleaching earth as catalyst for such a process at elevated temperature. U.S. Pat. No. 4,806,658 shows that polyethylene glycol can be hydrolyzed into ethylene glycol and its derivatives in the presence of metal oxide catalyst at high temperatures of from 170 to 320° C. Japan Pat. No. 60-109584 shows use of heteropoly acid as catalyst to depolymerize PTMEG. Japan 62-257931 shows use of non-crystalline silica-alumina as catalyst for depolymerization of linear or cyclic PTMEG at elevated temperature. Japan Pat. No. 11-269262 relates to a process where PTMEG is depolymerized over a mixture of zirconia and silica as catalyst.
Japan Pat. No. 61-11593 shows a method for depolymerization of cyclic PTMEG at elevated temperature over silicotungstic acid catalyst. WO No. 95/02625 shows use of metal perfluoroalkanesulfonates, for example (CF3SO3)3Y, as catalyst in the presence of an accelerator for a depolymerization mechanism. A distinct commercial disadvantage for this process is the high cost of the catalyst. Germany Pat. No. DE 4410685 shows depolymerizing PTMEG at elevated temperature over catalyst of kaolin, amorphous silica and/or zeolite X.
Pretreatment methods for THF to make same more useful as a polymerization feed include those shown in U.S. Pat. No. 4,189,566 where THF is treated with strong mineral acid, organosulfonic acid, silica gel or a bleaching earth; U.S. Pat. No. 4,803,299 where THF is fractionally distilled; and U.S. Pat. Nos. 6,403,842 and 6,987,201 THF polymerization color improvement focuses on purification of the carboxylic acid anhydride component.
None of the above publications show depolymerization of OCE resulting from copolymerization of THF and at least one other cyclic ether, which are very stable and more difficult to depolymerize than PTMEG. None of the above publications show a method for depolymerization of a mixture comprising OCE over a particular acid catalyst at suitable depolymerization reaction conditions including temperature and contact time in the presence of a specific depolymerization reaction enhancing additive to yield a reaction product comprising tetrahydrofuran. None of the above publications show a method for depolymerization of a mixture comprising OCE including steps to purify the depolymerization product producing high quality THF for PTMEG and its copolymer production.